A number of catalysts are known for the oxidation of unsaturated aldehydes to the corresponding acid. See, for example, U.S. Pat. No. 3,567,773. Although most of the disclosures suggest that the catalysts are useful for oxidizing methacrolein to methacrylic acid, it has been found that the yields of methacrylic acid are low and that special catalysts are required in this reaction.
U.S. Pat. No. 3,925,464 discloses a process for preparing unsaturated carboxylic acids which comprises reacting the corresponding unsaturated aldehydes with molecular oxygen in the presence of a catalyst consisting essentially of (a) molybdenum, (b) phosphorus, (c) at least one element selected from the group consisting of tungsten and magnesium, (d) at least one element selected from the group consisting of vanadium and bismuth, and (e) oxygen.